In particular, a local distribution grid has a plurality of branches, and each branch comprises at least one consumer and/or at least one decentralized power generator. A busbar connecting the plurality of branches of the local distribution grid with each other is connected with a controllable local distribution transformer that is also connected with the busbar via an electric line. Voltage measurement for the local distribution grid is provided via the busbar. The adjustment of an output voltage of the local distribution transformer can be carried out by an on-load tap changer.
The invention further relates to a method for controlling the stability of a local distribution grid by a controllable local distribution transformer.
For some time already, the parties involved have been paying attention to the direction of the energy flow, since the energy flow can be reversed in dependence on the present grid situation, weather conditions, present consumption, and the dimensioning of the feeders. This inevitably leads to problems with voltage compliance or with violations of the voltage bands, respectively, which voltage bands have to be complied with in accordance with standard EN 50160. This new development has to be observed and future energy structures will have to compensate for it, as the case may be. The grids constructed in the past, however, were not designed for a bidirectional energy flow.
In accordance with the directive (VDE-AR-N 4105) that came into force in the year 2012, PV inverters should supply the reactive power according to situation (fed active power). Consequently, both the capacitive and the inductive reactive power will play an increasingly important role for voltage compliance in the energy grid. So far, this issue has not been accounted for in any regulation algorithm.
In their mode of functioning, today's power grids are dimensioned to the load flow from the power plant to the consumer. The grid operator's quality management usually ensures that the quality criteria for voltage quality are complied with in accordance with DIN EN 50160 at all consumer connections of the respective grid.
The German patent application DE 10 2010 018 996 A1 discloses an apparatus and method for measuring the grid impedance of an electric supply network, in particular in a supply network, in which there either is no reference conductor or none is available. By providing an excitation signal to different interlinked phases, it is possible to measure a corresponding response signal. The grid impedance is calculated from the various excitation signals and the various response signals of the different interlinked phases.
A method for the adaptive regulation of the supply voltage in local distribution grids, into which electric energy, in particular in the form of solar energy or photovoltaic energy, is feedable and from which it is withdrawn by consumers, can be gathered from the German patent application DE 10 2011 108 255 A1. At least one transformer is used for regulation, which transformer is provided between a medium-voltage grid and a low-voltage grid. In order to carry out the regulation, different state variables of at least one of the feeders/consumers, such as, for instance, current, voltage, consumption, energy input, energy loss, or the like, are determined within the respective local distribution grid. For each phase, these state variables are fed to a regulator, in particular to a multivariable regulator. The respective regulator compares these state variables as input values against desired values/reference values and generates at least one regulating variable that is supplied to an electronically regulated transformer. Via the electronically regulated transformer, a regulated mains voltage deviating from the determined state variable is adjusted within specifiable steps in the respective local distribution grid, which mains voltage is measurable at a specifiable location of the local distribution grid.
The European patent application EP 2 592 709 A1 discloses a method for controlling the stability of a low-voltage grid, where an electric current is fed into a low-voltage grid by a local distribution transformer. The medium voltage is transformed to low voltage in the local distribution transformer. The low-voltage grid has a main power line, where electric current is conducted from the main power line via a grid branch point into at least one branch power line, with which at least one electricity consumer (consumer) and/or one electricity generator (feeder) is connected. The branch power line with the consumer and/or feeder connected therewith forms an autarkic grid district. At least one sensor is arranged at the grid branch point, at which the branch power line branches off from the main power line. The current flow in the branch power line and/or the voltage in the branch power line are measured by the sensor. An electricity withdrawal by the consumer from the branch power line and/or an energy input by the feeder into the branch power line and/or an electricity input from the main power line into the branch power line and/or the feed of the electric current from the local distribution transformer into the low-voltage grid is regulated on the basis of the current flow determined by the sensor and/or of the voltage determined by the sensor, with the provision that a branch power line overload is avoided in the grid district and/or that a voltage band is complied with in the grid district.
The international patent application WO 2011/076887 discloses a method for power regulation within a network segment of an alternating current network, which network segment has at least temporarily at least one electric storage. The network segment is connected via a transformer with a higher-level network level of the alternating current network, which level operates at a higher voltage than the network segment. Power fed in by the electric storage into the network segment and/or a power withdrawn by the electric storage from the network segment is carried out in dependence on an operating parameter of the network segment. It is provided that the power input is also carried out in dependence on a voltage difference between a feed voltage at a feed-in point of the electric storage and a transformer output voltage at a contact connected with a segment-side transformer output.
Based on the insights gained from the simulations, the information required for the quantitative assessment is generated, such as voltage fluctuations, currents and losses in the lines as well as the regulating requirements (switching intensity and switching quantity) of the on-load tap changer per year. This information provides a clear value added for the future regulating strategies; it is, for instance, possible to comply with the limit values set by the energy supply companies in a significantly more efficient manner.